Dude, you need both. I've never seen M, but have wanted it for quite some time. I do own The Seventh Seal, though, and I need to watch it again. It's kind of a weird movie, but it's #113 on IMDB.com's list of top 250 movies. M is #55, though.

I had been looking to see M for far longer and I get the feeling it will be a great example of Noir. I would choose M if you have to pick 1. Otherwise, go all out. I went crazy with this sale, because I doubt it will happen again any time soon and Criterion releases amazing movies.